With the increasing complexity and level of integration of electronic products, there is a growing need for distributed and independent power conversion devices, such as point-of-load voltage sources, to provide the well-regulated bias voltages for the highly integrated semiconductor devices that are commonly used. Highly integrated semiconductor devices frequently operate from specialized bias voltages. The power conversion devices must be economical and formed with very small dimensions to meet the size and portability needs of these markets, particularly markets that include portable and compact products such as cellular telephones and personal computers.
A power converter is conventionally formed with discrete magnetic devices such as transformers and inductors that are necessary in the design to achieve high power conversion efficiency. Such magnetic devices generally consist of electrically conductive windings, a body (“bobbin”) to support the windings, and a ferromagnetic core to provide a sufficiently high level of magnetic flux density for a given level of current in the windings. The assembly is generally mounted on a printed wiring board (“PWB”) for interconnection with other circuit elements.
A known technique to form magnetic devices is to employ planar windings that are formed directly in the buried metallic layers of a PWB. Exemplary magnetic cores that can be used with such structures are “EI” and “EE” core forms, so named for the corresponding shapes of the letters “E” and “I,” and produced by companies such as EPCOS and Philips. To insert and secure such cores to a PWB, apertures with complex and precise shapes must be milled in the PWB. Milling of such apertures in a PWB is generally a more costly mechanical operation than, for example, drilling of holes.
Power converters are also manufactured with discrete magnetic devices in an integrated circuit-size (“IC”) package, such as the DCR010505 power converter produced by Texas Instruments, Inc., to achieve a small physical size, as described by Geoff Jones in the paper entitled “Miniature Solutions for Voltage Isolation,” Texas Instruments' Analog Applications Journal, dated 3Q2005, pages 13-17. However, such power converters are generally larger than the needs of the more challenging markets for compact circuit devices, particularly for low power applications, and are produced at costs that do not meet the needs of high-volume production.
To produce inductors and transformers with small dimensions, studies have been undertaken to incorporate buried copper conductors within surrounding layers of a ferromagnetic film, for example, the study described by K. Yamaguchi, et al., in the paper entitled “Characteristic of a Thin Film Microtransformer with Circular Spiral Coils,” published in the IEEE Transaction on Magnetics, Vol. 29, No. 5, September 1993. The fabrication procedure described by Yamaguchi includes depositing a sputtered magnetic film that is then patterned using a photoresist. Copper windings are deposited by an electroplating process to produce a compact magnetic design. However, the overall process is not practical for a high-volume, low-cost production sequence in view of the complexity of the manufacturing steps that are necessary to produce a workable product.
Thus, there is a need for a process and related method to produce a magnetic device with very small physical dimensions that are adaptable to high volume and low cost manufacturing processes to meet the more challenging market needs that lie ahead that avoids the disadvantages of conventional approaches.